Asthma has become an epidemic affecting more than 155 million people in the world including ~22 million people in the United States. The chronic inflammation in asthma is due largely to the persistence of Th2 lymphocytes and Th2 cytokines/chemokines produced by both the structural cells of the lung as well as the infiltrating CD4+ lymphocytes, eosinophils, and basophils and the resident mast cells, leading to progressive loss of lung function. Recent studies have shown that IL-25 functions as an important mediator of Th2 responses and lies upstream of the classical Th2 cytokine responses. We recently reported that Act1, a novel E3 ubiquitin ligase, is an essential signaling molecule for IL-25 signaling, recruited to IL-25R upon ligand stimulation. Act1 deficiency in epithelial cells reduced IL-25-mediated allergic pulmonary inflammation, while Act1 deficiency in T cells also resulted in diminished Th2 responses and lung inflammation. Based on these findings, we hypothesize that the IL-25 induced Act1- mediated signaling pathway plays essential roles in Th2 cell responses and allergic pulmonary inflammation through the distinct impact on epithelial and T cell compartment. To test this hypothesis, we propose two Specific Aims: (1) Investigate the mechanistic role of IL-25-induced Act1-mediated signaling pathway in Th2 responses and allergic pulmonary inflammation;(2) Elucidate the molecular mechanism by which Act1 mediates IL-25 signaling and develop decoy (inhibitory) peptides as a potential therapeutic strategy for allergic pulmonary inflammation. The proposed research will provide significant insight into the events that both initiate and maintain the asthmatic phenotype, leading to increased potential to develop specific inhibitors of these pathways and novel therapeutics for the treatment of asthma. PUBLIC HEALTH RELEVANCE: Asthma has become an epidemic affecting more than 155 million people in the world including ~22 million people in the United States. Greater than 90% of childhood asthma and greater than 65% of adult asthma is the result of sensitization to a variety of environmental allergens (atopic asthma). The proposed study on IL-25 signaling will provide additional insight into the events that both initiate and maintain the asthmatic phenotype. With this improved understanding comes the potential to develop specific inhibitors of these pathways and perhaps novel therapeutics for the treatment of asthma.